1. Technical Field
The present disclosure relates to a preamplifier circuit for an acoustic transducer, in particular a MEMS (microelectromechanical system) capacitive microphone, to which the ensuing treatment will make explicit reference without this implying any loss of generality.
2. Description of the Related Art
As is known, there is a currently widespread use of acoustic transducers of a capacitive type, for example, MEMS capacitive microphones, in a wide range of applications, in particular within portable devices, in which it proves advantageous to reduce dimensions and levels of consumption.
A MEMS capacitive microphone generally comprises a microelectromechanical structure having a mobile electrode, provided as diaphragm or membrane, set facing a fixed electrode, to provide the plates of a variable-capacitance detection capacitor.
The mobile electrode is generally anchored to a substrate at a perimetral portion thereof, whilst a central portion thereof is free to move or bend in response to the pressure exerted by incident sound waves. The mobile electrode and the fixed electrode provide a capacitor, and bending of the membrane constituting the mobile electrode, as a function of the acoustic signal to be detected, causes a capacitance variation of this capacitor with respect to a value of capacitance at rest (which the capacitor assumes in the absence of acoustic signals).
In greater detail, and with reference to FIG. 1, a detection structure 1 of a MEMS capacitive microphone of a known type comprises a substrate 2 of semiconductor material, for example silicon; a cavity 3 (generally known as “back-chamber”) is made in the substrate 2, for example via chemical etching from the back. A membrane or diaphragm 4 is coupled to the substrate 2 and closes the back-chamber 3 at the top; the membrane 4 is flexible and, in use, undergoes deformation as a function of the pressure of the sound waves impinging thereon from the back-chamber 3. A rigid plate 5 (generally known as “back-plate”) is set above the membrane 4 and faces it, via the interposition of spacers 6 (for example, made of insulating material, such as silicon oxide). The back-plate 5 constitutes the fixed electrode of a capacitor with variable capacitance, the mobile electrode of which is constituted by the membrane 4, and has a plurality of holes 7, for example with circular cross section, which are designed to enable free circulation of air towards the same membrane 4.
Capacitive microphones, and in particular MEMS microphones, receive an appropriate electrical biasing so that they can be used as transducers of acoustic signals into electrical signals. In particular, in order to guarantee performance levels sufficient for common applications, microphones are required to be biased at high voltages (for example, 15 V-20 V), typically much higher than the ones at which a corresponding readout-interface circuit is supplied (logic voltages, for example, of 1.6 V-3 V). For this purpose, it is common to use charge-pump voltage-booster circuits made with integrated technology, which are capable of generating high voltage values starting from reference voltages of a lower value.
In use, the capacitance variations generated by the detection structure are transformed by a purposely provided readout-interface circuit into an electrical signal, which is supplied as output signal of the acoustic transducer. Since the capacitive variations are of an extremely low value (lower than one picofarad, generally in the femtofarad-picofarad range), the readout-interface circuit has a high signal-to-noise ratio in the conversion of the capacitive variations into the electrical signal to be used for the subsequent processing operations. In addition, portable applications have low supply voltages, for example in the region of 1.6 V or lower.
FIG. 2 shows a traditional circuit arrangement for reading of a MEMS capacitive microphone, the detection structure 1 of which is schematically represented as a capacitor with variable capacitance. This circuit arrangement, which is designed to operate as preamplifier of the capacitive variation signals generated by the detection structure, is, for example, described in Yu-Chun Hsu, Wen-Chieh Chou, Lu-Po Liao, Ji-Ching Tsai “A Realization of Low Noise Silicon Acoustic Transducer Interface Circuit”, VLSI Design, Automation and Test, 2007; VLSI-DAT 2007, International Symposium, Apr. 25-27, 2007, pp. 1-4.
In particular, a first terminal N1 (constituted, for example, by the back-plate 5—see FIG. 1) of the detection structure 1 receives a first biasing voltage VBIAS from an appropriate biasing circuit (typically comprising a charge-pump stage, not illustrated), whilst a second terminal N2 (for example, constituted by the membrane 4—see FIG. 1) of the detection structure 1 is connected to the high-impedance input of an associated preamplifier reading circuit (also defined as “front-end”), designated as a whole by 10.
The preamplifier circuit 10 includes a buffer stage 11, having an input connected to the aforesaid second terminal N2, and constituted by a stage in source-follower configuration, formed by a PMOS transistor 12. The PMOS transistor 12 has its gate terminal connected to the second terminal N2, its source terminal connected to a biasing-current generator 13, in turn connected to a line receiving a supply voltage VDD, and its drain terminal connected to a reference terminal (possibly coinciding, as in the case illustrated, with the ground terminal GND of the preamplifier circuit 10). The buffer stage 11 converts the capacitive variation signal generated by the detection structure 1 into an electrical voltage signal that can be used for the subsequent processing operations.
The input of the buffer stage 11 is biased at a fixed voltage through a resistance of a value sufficiently high as to guarantee the biasing charge on the detection structure 1 of the MEMS capacitive microphone to remain substantially fixed. A resistive biasing element 14, having a resistance of a high value, of the order of tens of gigaohms (or higher), is consequently connected between the input of the buffer stage 11 (the aforesaid second terminal N2) and a line at a second biasing voltage VREF (operating, for example, as a reference). Due to the fact that, as it is known, it is not possible in integrated-circuit technology to produce resistors with such high values of resistance, a pair of diodes in antiparallel configuration is usually employed to provide the resistive biasing element 14, which provide a sufficiently high resistance when there is a voltage drop across them of small value (depending upon the technology, for example less than 100 mV) and no d.c. current flows therein.
The preamplifier circuit 10 further comprises an amplification stage 15, provided with resistive feedback, connected in cascaded fashion to the output of the buffer stage 11. The amplification stage 15, in addition to implementing an appropriate gain function, performs the conversion of the single-ended signal coming from the buffer stage 11 into a differential signal between its two output terminals Out1, Out2. In greater detail, the amplification stage 15 comprises: an amplifier 16 having inverting and non-inverting inputs and two outputs connected to the aforesaid output terminals Out1, Out2; a first feedback resistor 18a, connected between the non-inverting input of the amplifier 16 and the first output terminal Out1; a second feedback resistor 18b, connected between the inverting input of the amplifier 16 and the second output terminal Out2 and having the same value of resistance as the first feedback resistor 18a; a first gain resistor 19a, connected between the non-inverting input of the amplifier 16 and the output of the buffer stage 11; and a second gain resistor 19b, connected between the inverting input of the amplifier 16 and the line at the second biasing voltage VREF.
Circuit solutions have also been proposed (see US 2008/152171), in which, instead of a follower stage at input to the reading chain, a gain stage of the common-emitter type is used. In this case, however, performance levels are limited by the reduced output dynamics of the gain stage.